There is a growing world wide demand for fertilizers containing element sulfur, since it was found that deficiencies of this element have created low crop yields. This sulfur deficiency in soils has developed over the last 20-30 years, due to a strict pollution control in most countries. Accordingly, the amount of SO.sub.2 has been drastically reduced in power station stacks, as well as due to the use of concentrated fertilizers containing little or no sulfur content.
There is a demand for a fertilizer containing about 10% sulfur, with maximum content of plant nutrients, based on water soluble P.sub.2 O.sub.5 (Triple Superphosphate), partially acidulated phosphate rock, monoammonium phosphate, diammonium phosphate and on phosphate rock. Generally, it is suggested that the sulfur should be in a finely dispersed state. A common accepted New Zealand specification states that about 50% should pass 100 mesh screen (150.mu.) and the coarsest fraction should be below 500.mu. for cool climates such as in New Zealand where much research was performed on the use of sulfur as a fertilizer. Elemental sulfur is preferred since it is not leached out of the soil as are sulfates, especially in humid climates, and since it is of a much higher concentration than other sulfur containing compounds. Sulfur with a particle size distribution as described above, has also slow release characteristics where the finer fraction is available during the first growing season, and the coarser fraction becomes available over the years. Elemental sulfur has some additional advantages in fertilizers:
it acts as a fungicide against certain microorganisms; PA1 it assists the decomposition of plant residues; PA1 it improves phosphorus utilization and reduces the pH of alkaline and calcareous soils.
There are a number of publications, including patents, which describe methods for obtaining sulfur-containing fertilizers. Various embodiments are claimed, but in principle most of the known methods are based on the incorporation of melted sulfur into the respective fertilizer. In this manner, the previously melted sulfur is solidified on the fertilizer particles. Optionally, the mixture is subsequently pelletized to impart to the granules some resistance to fracture. Thus according to the U.S. Pat. No. 2,097,446 the melted sulfur is mixed with phosphate rock particles and the resulted mixture is pelletized at a temperature in the range of between 220.degree. C. and 260.degree. C. A variation to this process is disclosed in the U.S. Pat. No. 2,161,035, wherein the bonding action of sulfur on the fertilizer particles is improved by the adjustment of the pH of the melted sulfur, by the addition of a mineral acid, prior to its mixing with the phosphate rock particles.
In the U.S. Pat. No. 3,177,062, it is mentioned that there are serious difficulties in the use of melted sulfur to obtain homogeneous fertilizer pellets or granules. Accordingly, a process is disclosed which avoids the use of melted sulfur. The process comprises blending pulverized phosphate rock with particulate elemental sulfur, granuating the resulting mixture to obtain a fraction with a size particles on the range of minus 10 to plus 20 mesh and subsequently stabilizing the granules by their heating at a temperature of about 150.degree. C.
In the U.S. Pat. No. 4,032,319 there are described composite fertilizers containing a non-uniform discontinuous, particulate core material in a water-permeable coating comprising sulfur. The method for their production involves the contact of the fertilizer particles with melted sulfur at a temperature at which the coating of said sulfur has an insufficient flow to form a continuous uniform coating, thus providing bare areas through which a hydrolytric attack takes place. Optionally, the coating contains a particulate modifier material which swells in the presence of water thereby promoting the release of the fertilizer Examples of such particulate modifier material are gypsum, potassium salts, bentonite, soluble salts of copper, zinc, borax, manganese and iron.
Among the disadvantages of the conventional methods it should be mentioned the requirement of melted sulfur, to use sulfur dust which is undesirable in its handling. Also, as mentioned in a review by H. P. Rothbaum et al (New Zealand Journal of Science, 1980, vol 23, 377) explosion hazards which might occur, are always due to sulfur dust which is inflammable. As known the use of melted sulfur itself carries with it the danger of fires in driers and explosions in dispatch areas. Another disadvantage is that upon the solidifying of the melted sulfur on the fertilizer particles, a broad range of particle sizes and even lumps might be formed together--or even separately--with the fertilizer particles. In order to obtain a more homogeneous mixture, a further grinding and screening might be required.
In another report by Charleston A. G. (Fert. Res. 1987, 11.2, 185-92) it is suggested to add melted sulfur to a solution of phosphoric acid to produce a slurry containing fine sulfur particles, to be used subsequently for the acidulation of phosphate rock.
The above brief review of some prior art references indicates that up to now there is no a simple method for obtaining sulfur-containing fertilizers which can be used without a further grinding and homogenization of the mixture obtained, operations which are relatively quite expensive and involve the danger of explosion. The problem has become recently more urgent, when a sulfur deficiency is noticed and this fertilizer nutrient began to be recognized as the fourth main element along the nitrogen, phosphorus and potassium elements. Also, according to the known methods, the losses of sulfur and its oxidation are unavoidable during the drying stage of the mixture. Other problems which have to be solved are connected with the handling of the powdered material, feeding it to the reactor and drying the final product.
It is an object of the present invention to provide a simple method for obtaining sulfur-containing fertilizers. It is another object of the present invention to provide a simple method for the utilization of waste materials for obtaining sulfur-containing fertilizers. It is another object of the present invention to provide a simple method for obtaining sulfur-containing fertilizers in which the sulfur is homogeneously distributed throughout the mass. It is another object of the present invention to obtain sulfur-containing fertilizers that do not give rise to dust during handling and storage and therefore are not subject to explosion and fire hazards. It is yet another object of the present invention to provide a simple method for obtaining sulfur-containing fertilizers which can be implemented in any fertilizer plant without requiring any accessory equipment for grinding the sulfur.